Structural approach to charge density waves in low-dimensional systems: electronic instability and chemical bonding.

The charge density wave (CDW) instability, usually occurring in low-dimensional metals, has been a topic of interest for longtime. However, some very fundamental aspects of the mechanism remain unclear. Recently, a plethora of new CDW materials, a substantial fraction of which is two-dimensional or even three-dimensional, has been prepared and characterised as bulk and/or single-layers. As a result, the need for revisiting the primary mechanism of the instability, based on the electron-hole instability established more than 50 years ago for quasi-one-dimensional (quasi-1D) conductors, has clearly emerged. In this work, we consider a large number of CDW materials to revisit the main concepts used in understanding the CDW instability, and emphasise the key role of the momentum dependent electron-phonon coupling in linking electronic and structural degrees of freedom. We argue that for quasi-1D systems, earlier weak coupling theories work appropriately and the energy gain due to the CDW and the concomitant periodic lattice distortion (PLD) remains primarily due to a Fermi surface nesting mechanism. However, for materials with higher dimensionality, intermediate and strong coupling regimes are generally at work and the modification of the chemical bonding network by the PLD is at the heart of the instability. We emphasise the need for a microscopic approach blending condensed matter physics concepts and state-of-the-art first-principles calculations with quite fundamental chemical bonding ideas in understanding the CDW phenomenon in these materials.

Basic aspects of the charge density wave instability of transition metal trichalcogenides NbSe3and monoclinic-TaS3.

NbSe3and monoclinic-TaS3(m-TaS3) are quasi-1D metals containing three different types of chains and undergoing two different charge density wave Peierls transitions atTP1andTP2associated with type III and type I chains, respectively. The nature of these transitions is discussed on the basis of first-principles DFT calculation of their Fermi surface (FS) and electron-hole response function. Because of the stronger inter-chain interactions, the FS and electron-hole response function are considerably more complex for NbSe3thanm-TaS3; however a common scenario can be put forward to rationalize the results. The intra-chain inter-band nesting processes dominate the strongest response for both type I and type III chains of the two compounds. Two well-defined maxima of the electron-hole response for NbSe3are found with the (0a*, 0c*) and (1/2a*, 1/2c*) transverse components atTP1andTP2, respectively, whereas the second maximum is not observed form-TaS3atTP2. Analysis of the different inter-chain coupling mechanisms leads to the conclusion that FS nesting effects are only relevant to set the transversea*components in NbSe3. The strongest inter-chain Coulomb coupling mechanism must be taken into account for the transverse coupling alongc*in NbSe3and along botha*andc*form-TaS3. Phonon spectrum calculations reveal the formation of a giant 2kFKohn anomaly form-TaS3. All these results support a weak coupling scenario for the Peierls transition of transition metal trichalcogenides.

Anion ordering transition and Fermi surface electron-hole instabilities in the (TMTSF)2ClO4 and (TMTSF)2NO3 Bechgaard salts analyzed through the first-principles Lindhard response function.

The first-principles electron-hole Lindhard response function has been calculated and analyzed in detail for two (TMTSF)2 X (X = ClO4 and NO3) Bechgaard salts undergoing different anion-ordering (AO) transitions. The calculation was carried out using the real triclinic low-temperature structures. The evolution of the electron-hole response with temperature for both relaxed and quenched salts is discussed. It is shown that the 2k F response of the quenched samples of both salts display a low temperature curved and tilted triangular continuum of maxima. This is not the case for the relaxed samples. (TMTSF)2ClO4 in the AO state exhibits a more quasi-1D response than in the non AO state and relaxed (TMTSF)2NO3 shows a sharp maximum. The curved triangular plateau of the quenched samples results from multiple nesting of the warped quasi-1D Fermi surface which implies the existence of a large q range of electron-hole fluctuations. This broad maxima region is around 1% of the Brillouin zone area for the X = ClO4 salt (and X = PF6) but only 0.1% for the X = NO3 salt. It is suggested that the strong reduction of associated SDW fluctuations could explain the non detection of the SDW-mediated superconductivity in (TMTSF)2NO3. The calculated maxima of the Lindhard response nicely account for the modulation wave vector experimentally determined by NMR in the SDW ground state of the two salts. The critical AO wave vector for both salts is located in regions where the Lindhard response is a minimum so that they are unrelated to any electron-hole instability. The present first-principles calculation reveals 3D effects in the Lindhard response of the two salts at low temperature which are considerably more difficult to model in analytical approaches.

New insights into the structural properties of κ-(BEDT-TTF)2Ag2(CN)3 spin liquid.

Here, the first accurate study is presented of the room-temperature and 100 K structures of one of the first organic spin liquids, κ-(BEDT-TTF)2Ag2(CN)3. It is shown that the monoclinic structure determined previously is only the average one. It is shown that the exact structure presents triclinic symmetry with two non-equivalent dimers in the unit cell. But surprisingly this does not lead to a sizeable charge disproportionation between dimers. The difference from the analogue compound κ-(BEDT-TTF)2Cu2(CN)3 which also presents a spin liquid phase is discussed in detail. The data provided here show the importance of the anionic layer and in particular the transition metal position in the process of symmetry breaking. The possible impact of the symmetry breaking, albeit weak, on the spin-liquid mechanism and the influence of various disorders on the physical properties of this system is also discussed.

Fermi surface electron-hole instability of the (TMTSF)2PF6 Bechgaard salt revealed by the first-principles Lindhard response function.

We report the first-principles DFT calculation of the electron-hole Lindhard response function of the (TMTSF)2PF6 Bechgaard salt using the real triclinic low-temperature structure. The Lindhard response is found to change considerably with temperature. Near the 2k F spin density wave (SDW) instability it has the shape of a broad triangular plateau as a result of the multiple nesting associated with the warped quasi-one-dimensional Fermi surface. The evolution of the 2k F broad maximum as well as the effect of pressure and deuteration is calculated and analyzed. The thermal dependence of the electron-hole coherence length deduced from these calculations compares very well with the experimental thermal evolution of the 2k F bond order wave correlation length. The existence of a triangular plateau of maxima in the low-temperature electron-hole Lindhard response of (TMTSF)2PF6 should favor a substantial mixing of q-dependent fluctuations which can have important consequences in understanding the phase diagram of the 2k F SDW ground state, the mechanism of superconductivity and the magneto-transport of this paradigmatic quasi-one-dimensional material. The first-principles DFT Lindhard response provides a very accurate and unbiased approach to the low-temperature instabilities of (TMTSF)2PF6 which can take into account in a simple way 3D effects and subtle structural variations, thus providing a very valuable tool in understanding the remarkable physics of molecular conductors.

Decoupling anion-ordering and spin-Peierls transitions in a strongly one-dimensional organic conductor with a chessboard structure, (o-Me2TTF)2NO3.

A mixed-valence conducting cation radical salt of the unsymmetrically substituted o-Me2TTF donor molecule (TTF is tetrathiafulvalene) was obtained upon electrocrystallization in the presence of the non-centrosymmetric NO3- anion. It crystallizes at room temperature in the monoclinic P21/c space group, with the anion disordered on an inversion centre. The donor molecules are stacked along the a axis. A 90° rotation of the longest molecular axis of o-Me2TTF generates a chessboard-like structure, preventing lateral S⋯S contacts between stacks and providing a strongly one-dimensional electronic system, as confirmed by overlap interaction energies and band structure calculations. A strong dimerization within the stacks explains the semi-conducting behaviour of the salt, with σroom temp = 3-5 S cm-1 and Eactivated = 0.12-0.14 eV. An X-ray diffuse scattering survey of reciprocal space, combined with full structure resolutions at low temperatures (250, 85 and 20 K), evidenced the succession of two structural transitions: a ferroelastic one with an anion-ordering (AO) process and the establishment of a (0, ½, ½) superstructure below 124 (±3) K, also visible via resistivity thermal dependence, followed by a stack tetramerization with the establishment of a (½, ½, ½) superstructure below 90 (±5) K. The latter ground state is driven by a spin-Peierls (SP) instability, as demonstrated by the temperature dependence of the magnetic susceptibility. Surprisingly, these two kinds of instability appear to be fully decoupled here, at variance with other tetra-methyl-tetra-thia-fulvalene (TMTTF) or tetramethyl-tetra-selena-fulvalene (TMTSF) salts with such non-centrosymmetric counter-ions.

Structural and electronic control of the metal to insulator transition and local orderings in the θ-(BEDT-TTF)2X organic conductors.

A first-principles density functional theory (DFT) study of [Formula: see text]-(BEDT-TTF)2X molecular conductors with X = I3, CsCo(SCN)4 (ambient pressure, 7.5 kbar and 10 kbar), CsZn(SCN)4, TlCo(SCN)4, RbCo(SCN)4 and RbZn(SCN)4 (220 K and 90 K) is reported. It is shown that these salts exhibit three different types of band structure each of them associated with a different physical behavior. In contrast with previous proposals it is found that the key electronic parameter behind the differences in the band structures is the intrastack transfer integral, t c . A new mechanism for the metal to insulator transition in the [Formula: see text]-(BEDT-TTF)2MM'(SCN)4 ([Formula: see text], Tl; [Formula: see text], Co) salts is proposed, where an order-disorder structural transition of the ethylenedithio groups doubling the periodicity along the stack direction drives the system into an electronically pseudo-1D system along the interstack direction that is subject to a 4k F charge localization of holes. The structural rearrangement is such that the holes are not distributed equally between the two donors; the larger hole density is associated with the B donors which establish the strongest hydrogen bonds with the anion layers. A detailed microscopic description of how disorder of the ethylenedithio groups, the θ dihedral angle and the electronic structure intermingle and lead to the unusual phase diagram of these salts is presented. In this framework the role of pressure and uniaxial strain in controlling the physical behavior of these salts is discussed.

Charge-ordering transition in (TMTTF)2X explored via dilatometry.

Charge-ordering phenomena have been highly topical over the past few years. A phase transition towards a charge-ordered state has been observed experimentally in several classes of materials. Among them, many studies have been devoted to the family of quasi-one-dimensional organic charge-transfer salts (TMTTF)2X, where (TMTTF) stands for tetramethyltetrathiafulvalene and X for a monovalent anion (X = PF6, AsF6 and SbF6). However, the relationship between the electron localization phenomena and the role of the lattice distortion in stabilizing the charge-ordering pattern is poorly documented in the literature. Here we present a brief overview of selected literature results, with emphasis placed on recent thermal expansion experiments probing the charge-ordering transition of these salts.

Concerning the possibility of hidden one-dimensional Fermi surfaces for the K(0.25)WO(3) hexagonal bronze.

On the basis of first-principles density functional theory calculations, it is shown that, in contrast with a recent suggestion, K(0.25)WO(3) does not exhibit hidden one-dimensional Fermi surfaces. A charge-density-wave instability should not be at the origin of the resistivity anomaly of this bronze, which most likely results from potassium vacancy ordering.

One-dimensional instability in BaVS3.

The 3d(1) system BaVS3 undergoes a series of remarkable electronic phase transitions. We show that the metal-insulator transition at T(MI)=70 K is associated with a structural transition announced by a huge regime of one-dimensional (1D) lattice fluctuations, detected up to 170 K. These 1D fluctuations correspond to a 2k(F)=c(*)/2 charge-density wave (CDW) instability of the d(z(2)) electron gas. We discuss the formation below T(MI) of an unconventional CDW state involving the condensation of the other V4+ 3d(1) electrons of the quasidegenerate e(t(2g)) orbitals. This study stresses the role of the orbital degrees of freedom in the physics of BaVS3 and reveals the inadequacy of current first principle band calculations to describe its electronic ground state.